Coal contributes to a large percentage of the electricity generation in the world today and is expected to maintain its dominant share in the foreseeable future. Nonetheless, significant environmental pressures have led to increased environmental demands requiring, not only high efficiency but also reduced emission levels of CO2, SO2, NOx, and mercury to ultra-low levels.
A particular advantageous plant arrangement is the use of an Oxy-combustion steam plant with CO2 capture. Oxy-combustion systems use oxygen, usually produced in an air separation unit, instead of air, for the combustion of the primary fuel. The oxygen is often mixed with an inert gas, such as recirculated flue gas, in order to keep the combustion temperature at a suitable level. In addition, it may be preferable to separately or additionally preheat oxygen from the Air Separation Unit in a steam coil oxygen preheater.
Oxy-combustion processes produce flue gas having CO2, water and O2 as its main constituents, the CO2 concentration being typically greater than about 70% by volume. Therefore, CO2 capture from the flue gas of an oxy-combustion process can be done relatively simply in a Gas Processing Unit.
An example of a typical water steam cycle of a high efficiency oxy-combustion steam plants is shown in FIG. 1. The plant comprises a triple-pressure series of reheat steam turbines HP, IP, LP fed by steam from a boiler 142. Exhaust steam from the last low pressure steam turbine LP is condensed in a condenser 102 before being polished 104 and pumped via a condensate pump 103 successively through a series of low pressure heater 106,107,108,109,131, a feed water tank 136 and high pressure heaters 132 before returning to the boiler 142 in a closed loop. The heat source for the low and high pressure heaters is typically steam extracted from the low/intermediate and high pressure steam turbines.
Due to the large benefit in ensuring the highest efficiency cycle there is a continuing need to find ways of better integrating the thermal needs and sinks of the oxy-combustion capture systems within the steam power plant. This requires an optimization of the heat needs and sinks of the capture system with the plant cycle to ensure no energy is wasted. In particular, this needs consideration of how to integrate the steam coil oxygen preheater into the thermal cycle.